The present invention relates to a method and a device for cooling a unit using a cold head, with a thermal cooling of the unit to be cooled by means of the thermosiphon principle.
Cold heads are frequently used to cool units, such as for instance superconducting coils, down to low temperatures. Superconducting coils can be used for instance in magnetic spin tomography devices, in motors, in generators or in current limiters. Cooling occurs here to temperatures of below 100 K. Particularly with the use of high-temperature superconducting (HTS)-materials, such as for instance Y2BaCu3O7 (YBCO), superconducting properties of the conductor are already achieved at temperatures of liquid nitrogen.
Demands placed on the cooling system when cooling a unit are inter alia a short cooling-down time, a low temperature gradient within the unit to be cooled and/or a small temperature difference between the cold head and the unit to be cooled. The cooling of a unit with the aid of a cold head is described below. This is naturally similarly understood to mean the cooling of a number of units with a cold head, a unit with a number of cold heads or a number of units with a number of cold heads.
In order to cool a unit with a cold head, the unit to be cooled must be thermally connected to the cold head. There are different methods of thermally coupling the cold head to the unit to be cooled. Therefore the unit to be cooled can either be thermally coupled to the cold head with the aid of a heat bridge via heat conductance. Alternatively, the prior art discloses a thermal coupling of the unit to be cooled with the aid of a thermosiphon.
With the thermal coupling of the unit to be cooled with the aid of a heat bridge, the cold head is connected to the unit to be cooled by way of copper rails or copper bands. Since the cold head is connected via heat conductance with the unit to be cooled, the cold head is kept at a temperature T during the cooling-down process, which lies relatively just below the temperature TE of the unit. The temperature difference is dependent on the length and on the cross-section of the connection between the cold head and the unit to be cooled.
The cooling power P of a cold head reduces with the cold head temperature T. The minimal temperature difference between the cold head and the unit to be cooled causes the cold head always to operate at a high cooling power P within an optimal temperature range when cooling-down the system. The unit to be cooled can thus be cooled down relatively quickly.
Correspondingly large material cross-sections of the heat bridge are required to ensure that no large temperature gradients occur in the unit to be cooled and at the cold head in the cooled-down state. As a result, this can result in impermissibly high mechanical stresses on the sensitive cold head. Large temperature gradients in the cooled-down state in the unit to be cooled and at the cold head are to be prevented, since these result in a poor degree of efficiency during the cooling process.
During the thermal coupling of the unit to be cooled with the aid of a thermosiphon, a gaseous fluid, in particular neon, is condensed in a condenser. The condenser is to be connected to the cold head in an effective heat-conducting manner. The liquid fluid flows toward the unit to be cooled and can absorb heat there by transferring into the gaseous state. Since the condensation and evaporation of the fluid in the entire system almost takes place at the same temperature, only very small temperature gradients occur within the unit to be cooled and at the cold head. As a result the operating temperature of the cold head always lies at the boiling temperature of the cooling medium used.
Since the cold head also lies at a lower temperature T during the entire cooling-down period, it only supplies a relatively low cooling power P during this time. This causes the cooling-down of the object to be cooled to take a relatively long period of time.